Interpretive Summary: Sunflower is a plant of the American Continent, but now many wild species are spread in different part of the world. Along the southeastern coast of Africa, two sunflower species, H. argophyllus and H. debilis, possibly both of Texas origin, grow far apart. However in two sites on sandy soil they grow together and many plants share morphological traits typical of the two species. A previous study, carried out by AFLP markers and morphological observations, demonstrated an introgression of H. debilis germplasm into H. argophyllus. We hypothesise that seeds of the two species were accidentally introduced into Africa by man from the area of origin of sunflowers. To obtain further evidences of this introgression process and to investigate the origin of these populations, we compared African populations with a set of American populations of the two species from Texas, using 20 nuclear H. annuus SSRs. To facilitate the study on interpopulational gene flow and introgressive hybridization events, uni-parental inherited markers were also considered and ten universal chloroplast SSRs tested. The analyses were carried out on 68 and 137 plants of H. argophyllus sampled in Texas and Africa, respectively, and 26 hybrids isolated in the Inhambane area, Mozambique. Six chloroplast SSRs and 13 nuclear SSRs worked, and 6 and 4, respectively, were polymorphic. A preliminary test on one cpSSR locus shows a putative bottleneck in the African population of H. argophyllus. The two species and the hybrids share the same alleles at all cpSSR loci, indicating a very recent divergence and/or a sustained gene flow. Nuclear loci show a high level of variability. In some cases allele size range of African populations of the two species are more similar to each other than to their respective American couterparts, also suggesting a high level of gene flow. To ascertain the introgression process, genetic and morphological data need to be directly compared in natural populations and controlled crosses. For this purpose interspecific bi-parental crosses are in progress between H. argophyllus and H. debilis.

Technical Abstract:
Sunflower is a plant of the American Continent, but now many wild species are spread in different part of the world. Along the southeastern coast of Africa, two sunflower species, H. argophyllus and H. debilis, possibly both of Texas origin, grow far apart. However in two sites on sandy soil they grow together and many plants share morphological traits typical of the two species. A previous study, carried out by AFLP markers and morphological observations, demonstrated an introgression of H. debilis germplasm into H. argophyllus. We hypothesise that seeds of the two species were accidentally introduced into Africa by man from the area of origin of sunflowers. To obtain further evidences of this introgression process and to investigate the origin of these populations, we compared African populations with a set of American populations of the two species from Texas, using 20 nuclear H. annuus SSRs. To facilitate the study on interpopulational gene flow and introgressive hybridization events, uni-parental inherited markers were also considered and ten universal chloroplast SSRs tested. The analyses were carried out on 68 and 137 plants of H. argophyllus sampled in Texas and Africa, respectively, and 26 hybrids isolated in the Inhambane area, Mozambique. Six chloroplast SSRs and 13 nuclear SSRs worked, and 6 and 4, respectively, were polymorphic. A preliminary test on one cpSSR locus shows a putative bottleneck in the African population of H. argophyllus. The two species and the hybrids share the same alleles at all cpSSR loci, indicating a very recent divergence and/or a sustained gene flow. Nuclear loci show a high level of variability. In some cases allele size range of African populations of the two species are more similar to each other than to their respective American couterparts, also suggesting a high level of gene flow. To ascertain the introgression process, genetic and morphological data need to be directly compared in natural populations and controlled crosses. For this purpose interspecific bi-parental crosses are in progress between H. argophyllus and H. debilis.